Update revision history

* added hash_is_avalanching

* launched CI after enabling GHA

* moved 1.89 entry from Change Log to Recent Changes

* segregated some tests into hash_is_avalanching_test3.cpp and gotten rid of Unordered dependency

* removed unneeded include

* stopped using external std::hash for testing

* typo

* removed left over include

* typo

* moved hash_is_avalanching from boost::container_hash to boost

* fixed specializations of boost::hash_is_avalanching

.drone.jsonnet

@@ -34,7 +34,6 @@ local linux_pipeline(name, image, environment, packages = "", sources = [], arch
                 'set -e',
                 'uname -a',
                 'echo $DRONE_STAGE_MACHINE',
-                'wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -',
             ] +
             (if sources != [] then [ 'apt-get update' ] + [ ('apt-add-repository "' + source + '"') for source in sources ] else []) +
             (if packages != "" then [ 'apt-get update', 'apt-get -y install ' + packages ] else []) +
@@ -103,7 +102,6 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
         "cppalliance/droneubuntu1404:1",
         { TOOLSET: 'gcc', COMPILER: 'g++-4.7', CXXSTD: '0x', ADDRMD: '32,64' },
         "g++-4.7-multilib",
-        [ "ppa:ubuntu-toolchain-r/test" ],
     ),
 
     linux_pipeline(
@@ -113,11 +111,10 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
     ),
 
     linux_pipeline(
-        "Linux 14.04 GCC 4.9 32/64",
+        "Linux 16.04 GCC 4.9 32/64",
-        "cppalliance/droneubuntu1404:1",
+        "cppalliance/droneubuntu1604:1",
         { TOOLSET: 'gcc', COMPILER: 'g++-4.9', CXXSTD: '11', ADDRMD: '32,64' },
         "g++-4.9-multilib",
-        [ "ppa:ubuntu-toolchain-r/test" ],
     ),
 
     linux_pipeline(
@@ -187,15 +184,15 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
     ),
 
     linux_pipeline(
-        "Linux 23.04 GCC 13 32/64 UBSAN",
+        "Linux 24.04 GCC 13 32/64 UBSAN",
-        "cppalliance/droneubuntu2304:1",
+        "cppalliance/droneubuntu2404:1",
         { TOOLSET: 'gcc', COMPILER: 'g++-13', CXXSTD: '11,14,17,20,2b', ADDRMD: '32,64' } + ubsan,
         "g++-13-multilib",
     ),
 
     linux_pipeline(
-        "Linux 23.04 GCC 13 32 ASAN",
+        "Linux 24.04 GCC 13 32 ASAN",
-        "cppalliance/droneubuntu2304:1",
+        "cppalliance/droneubuntu2404:1",
         { TOOLSET: 'gcc', COMPILER: 'g++-13', CXXSTD: '11,14,17,20,2b', ADDRMD: '32' } + asan,
         "g++-13-multilib",
     ),
@@ -214,6 +211,27 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
         "g++-14-multilib",
     ),
 
+    linux_pipeline(
+        "Linux 26.04 GCC 15 UBSAN",
+        "cppalliance/droneubuntu2604:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-15', CXXSTD: '11,14,17,20,23,2c' } + ubsan,
+        "g++-15-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 26.04 GCC 15 ASAN",
+        "cppalliance/droneubuntu2604:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-15', CXXSTD: '11,14,17,20,23,2c' } + asan,
+        "g++-15-multilib",
+    ),
+
+    linux_pipeline(
+        "Linux 26.04 GCC 16",
+        "cppalliance/droneubuntu2604:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-16', CXXSTD: '11,14,17,20,23,2c' },
+        "g++-16-multilib",
+    ),
+
     linux_pipeline(
         "Linux 16.04 Clang 3.5",
         "cppalliance/droneubuntu1604:1",
@@ -334,38 +352,59 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
     ),
 
     linux_pipeline(
-        "Linux 23.04 Clang 16",
+        "Linux 24.04 Clang 16",
-        "cppalliance/droneubuntu2304:1",
+        "cppalliance/droneubuntu2404:1",
         { TOOLSET: 'clang', COMPILER: 'clang++-16', CXXSTD: '11,14,17,20,2b' },
         "clang-16",
     ),
 
     linux_pipeline(
-        "Linux 23.10 Clang 17 UBSAN",
+        "Linux 24.04 Clang 17",
-        "cppalliance/droneubuntu2310:1",
+        "cppalliance/droneubuntu2404:1",
-        { TOOLSET: 'clang', COMPILER: 'clang++-17', CXXSTD: '11,14,17,20,2b' } + ubsan,
+        { TOOLSET: 'clang', COMPILER: 'clang++-17', CXXSTD: '11,14,17,20,2b' },
         "clang-17",
     ),
 
     linux_pipeline(
-        "Linux 23.10 Clang 17 ASAN",
+        "Linux 24.04 Clang 18",
-        "cppalliance/droneubuntu2310:1",
-        { TOOLSET: 'clang', COMPILER: 'clang++-17', CXXSTD: '11,14,17,20,2b' } + asan,
-        "clang-17",
-    ),
-
-    linux_pipeline(
-        "Linux 24.04 Clang 18 UBSAN",
         "cppalliance/droneubuntu2404:1",
-        { TOOLSET: 'clang', COMPILER: 'clang++-18', CXXSTD: '11,14,17,20,2b' } + ubsan,
+        { TOOLSET: 'clang', COMPILER: 'clang++-18', CXXSTD: '11,14,17,20,2b' },
         "clang-18",
     ),
 
     linux_pipeline(
-        "Linux 24.04 Clang 18 ASAN",
+        "Linux 24.04 Clang 19",
         "cppalliance/droneubuntu2404:1",
-        { TOOLSET: 'clang', COMPILER: 'clang++-18', CXXSTD: '11,14,17,20,2b' } + asan,
+        { TOOLSET: 'clang', COMPILER: 'clang++-19', CXXSTD: '11,14,17,20,2b' },
-        "clang-18",
+        "clang-19",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 Clang 20 UBSAN",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-20', CXXSTD: '11,14,17,20,23,2c' } + ubsan,
+        "clang-20",
+    ),
+
+    linux_pipeline(
+        "Linux 24.04 Clang 20 ASAN",
+        "cppalliance/droneubuntu2404:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-20', CXXSTD: '11,14,17,20,23,2c' } + asan,
+        "clang-20",
+    ),
+
+    linux_pipeline(
+        "Linux 26.04 Clang 21",
+        "cppalliance/droneubuntu2604:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-21', CXXSTD: '11,14,17,20,23,2c' },
+        "clang-21",
+    ),
+
+    linux_pipeline(
+        "Linux 26.04 Clang 22",
+        "cppalliance/droneubuntu2604:1",
+        { TOOLSET: 'clang', COMPILER: 'clang++-22', CXXSTD: '11,14,17,20,23,2c' },
+        "clang-22",
     ),
 
     macos_pipeline(
@@ -413,4 +452,10 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
         "cppalliance/dronevs2022:1",
         { TOOLSET: 'msvc-14.3', CXXSTD: '14,17,20,latest' },
     ),
+
+    windows_pipeline(
+        "Windows VS2026 msvc-14.5",
+        "cppalliance/dronevs2026:1",
+        { TOOLSET: 'msvc-14.5', CXXSTD: '14,17,20,latest' },
+    ),
 ]

.github/workflows/ci.yml

@@ -17,116 +17,108 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - toolset: gcc-4.8
-            cxxstd: "11"
-            os: ubuntu-latest
-            container: ubuntu:18.04
-            install: g++-4.8-multilib
-            address-model: 32,64
-          - toolset: gcc-5
-            cxxstd: "11,14,1z"
-            os: ubuntu-latest
-            container: ubuntu:18.04
-            install: g++-5-multilib
-            address-model: 32,64
-          - toolset: gcc-6
-            cxxstd: "11,14,1z"
-            os: ubuntu-latest
-            container: ubuntu:18.04
-            install: g++-6-multilib
-            address-model: 32,64
           - toolset: gcc-7
             cxxstd: "11,14,17"
-            os: ubuntu-20.04
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: g++-7-multilib
             address-model: 32,64
           - toolset: gcc-8
             cxxstd: "11,14,17,2a"
-            os: ubuntu-20.04
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: g++-8-multilib
             address-model: 32,64
           - toolset: gcc-9
             cxxstd: "11,14,17,2a"
-            os: ubuntu-20.04
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: g++-9-multilib
             address-model: 32,64
           - toolset: gcc-10
             cxxstd: "11,14,17,2a"
-            os: ubuntu-22.04
+            container: ubuntu:22.04
+            os: ubuntu-latest
             install: g++-10-multilib
             address-model: 32,64
           - toolset: gcc-11
             cxxstd: "11,14,17,20"
-            os: ubuntu-22.04
+            container: ubuntu:22.04
+            os: ubuntu-latest
             install: g++-11-multilib
             address-model: 32,64
           - toolset: gcc-12
             cxxstd: "11,14,17,20,2b"
-            os: ubuntu-22.04
+            container: ubuntu:22.04
+            os: ubuntu-latest
             install: g++-12-multilib
             address-model: 32,64
           - toolset: gcc-13
             cxxstd: "11,14,17,20,2b"
+            container: ubuntu:24.04
             os: ubuntu-latest
-            container: ubuntu:23.04
             install: g++-13-multilib
             address-model: 32,64
           - toolset: gcc-14
             cxxstd: "11,14,17,20,2b"
-            os: ubuntu-latest
             container: ubuntu:24.04
+            os: ubuntu-latest
             install: g++-14-multilib
             address-model: 32,64
-          - toolset: clang
+          - toolset: gcc-15
-            compiler: clang++-3.9
+            cxxstd: "11,14,17,20,23,2c"
-            cxxstd: "11,14"
+            container: ubuntu:26.04
             os: ubuntu-latest
-            container: ubuntu:18.04
+            install: g++-15-multilib
-            install: clang-3.9
+            address-model: 32,64
-          - toolset: clang
+          - toolset: gcc-16
-            compiler: clang++-4.0
+            cxxstd: "11,14,17,20,23,2c"
-            cxxstd: "11,14"
+            container: ubuntu:26.04
             os: ubuntu-latest
-            container: ubuntu:18.04
+            install: g++-16-multilib
-            install: clang-4.0
+            address-model: 32,64
-          - toolset: clang
-            compiler: clang++-5.0
-            cxxstd: "11,14,1z"
-            os: ubuntu-latest
-            container: ubuntu:18.04
-            install: clang-5.0
           - toolset: clang
             compiler: clang++-6.0
             cxxstd: "11,14,17"
-            os: ubuntu-20.04
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-6.0
           - toolset: clang
             compiler: clang++-7
             cxxstd: "11,14,17"
-            os: ubuntu-20.04
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-7
           - toolset: clang
             compiler: clang++-8
             cxxstd: "11,14,17"
-            os: ubuntu-20.04
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-8
           - toolset: clang
             compiler: clang++-9
             cxxstd: "11,14,17,2a"
-            os: ubuntu-20.04
+            container: ubuntu:20.04
+            os: ubuntu-latest
             install: clang-9
           - toolset: clang
             compiler: clang++-10
             cxxstd: "11,14,17,2a"
-            os: ubuntu-20.04
+            container: ubuntu:20.04
+            os: ubuntu-latest
+            install: clang-10
           - toolset: clang
             compiler: clang++-11
             cxxstd: "11,14,17,2a"
-            os: ubuntu-20.04
+            container: ubuntu:20.04
+            os: ubuntu-latest
+            install: clang-11
           - toolset: clang
             compiler: clang++-12
             cxxstd: "11,14,17,20"
-            os: ubuntu-20.04
+            container: ubuntu:20.04
+            os: ubuntu-latest
+            install: clang-12
           - toolset: clang
             compiler: clang++-13
             cxxstd: "11,14,17,20,2b"
@@ -148,13 +140,13 @@ jobs:
           - toolset: clang
             compiler: clang++-16
             cxxstd: "11,14,17,20,2b"
-            container: ubuntu:23.04
+            container: ubuntu:24.04
             os: ubuntu-latest
             install: clang-16
           - toolset: clang
             compiler: clang++-17
             cxxstd: "11,14,17,20,2b"
-            container: ubuntu:23.10
+            container: ubuntu:24.04
             os: ubuntu-latest
             install: clang-17
           - toolset: clang
@@ -164,14 +156,38 @@ jobs:
             os: ubuntu-latest
             install: clang-18
           - toolset: clang
+            compiler: clang++-19
             cxxstd: "11,14,17,20,2b"
-            os: macos-12
+            container: ubuntu:24.04
+            os: ubuntu-latest
+            install: clang-19
           - toolset: clang
-            cxxstd: "11,14,17,20,2b"
+            compiler: clang++-20
-            os: macos-13
+            cxxstd: "11,14,17,20,23,2c"
+            container: ubuntu:24.04
+            os: ubuntu-latest
+            install: clang-20
+          - toolset: clang
+            compiler: clang++-21
+            cxxstd: "11,14,17,20,23,2c"
+            container: ubuntu:26.04
+            os: ubuntu-latest
+            install: clang-21
+          - toolset: clang
+            compiler: clang++-22
+            cxxstd: "11,14,17,20,23,2c"
+            container: ubuntu:26.04
+            os: ubuntu-latest
+            install: clang-22
           - toolset: clang
             cxxstd: "11,14,17,20,2b"
             os: macos-14
+          - toolset: clang
+            cxxstd: "11,14,17,20,2b"
+            os: macos-15
+          - toolset: clang
+            cxxstd: "11,14,17,20,23,2c"
+            os: macos-26
 
     runs-on: ${{matrix.os}}
     container: ${{matrix.container}}
@@ -181,17 +197,13 @@ jobs:
         shell: bash
 
     steps:
-      - name: Enable Node 16
-        run: |
-          echo "ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true" >> $GITHUB_ENV
-
-      - uses: actions/checkout@v3
-
       - name: Setup container environment
         if: matrix.container
         run: |
           apt-get update
-          apt-get -y install sudo python3 git g++
+          apt-get -y install sudo python3 git g++ curl xz-utils
+
+      - uses: actions/checkout@v6
 
       - name: Install packages
         if: matrix.install
@@ -238,31 +250,27 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - toolset: msvc-14.0
-            cxxstd: 14,latest
-            addrmd: 32,64
-            os: windows-2019
-          - toolset: msvc-14.2
-            cxxstd: "14,17,20,latest"
-            addrmd: 32,64
-            os: windows-2019
           - toolset: msvc-14.3
             cxxstd: "14,17,20,latest"
             addrmd: 32,64
             os: windows-2022
-          - toolset: clang-win
+          - toolset: msvc-14.5
-            cxxstd: "14,17,latest"
+            cxxstd: "14,17,20,latest"
             addrmd: 32,64
-            os: windows-2022
+            os: windows-2025-vs2026
+          - toolset: clang-win
+            cxxstd: "14,17,20,latest"
+            addrmd: 32,64
+            os: windows-2025
           - toolset: gcc
             cxxstd: "11,14,17,2a"
             addrmd: 64
-            os: windows-2019
+            os: windows-2025
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Boost
         shell: cmd
@@ -298,16 +306,13 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-20.04
+          - os: ubuntu-latest
-          - os: ubuntu-22.04
+          - os: macos-latest
-          - os: macos-12
-          - os: macos-13
-          - os: macos-14
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install packages
         if: matrix.install
@@ -346,16 +351,13 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-20.04
+          - os: ubuntu-latest
-          - os: ubuntu-22.04
+          - os: macos-latest
-          - os: macos-12
-          - os: macos-13
-          - os: macos-14
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install packages
         if: matrix.install
@@ -404,16 +406,13 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-20.04
+          - os: ubuntu-latest
-          - os: ubuntu-22.04
+          - os: macos-latest
-          - os: macos-12
-          - os: macos-13
-          - os: macos-14
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install packages
         if: matrix.install
@@ -460,13 +459,12 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: windows-2019
+          - os: windows-latest
-          - os: windows-2022
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Boost
         shell: cmd
@@ -509,13 +507,12 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: windows-2019
+          - os: windows-latest
-          - os: windows-2022
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Boost
         shell: cmd
@@ -576,13 +573,12 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: windows-2019
+          - os: windows-latest
-          - os: windows-2022
 
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Boost
         shell: cmd

README.md

@@ -8,7 +8,7 @@ by C++11 as `std::hash`, and several support facilities (`hash_combine`,
 
 `boost::hash` supports most standard types and some user-defined types out of
 the box, and is extensible; it's possible for a user-defined type `X` to make
-iself hashable via `boost::hash<X>` by defining an appropriate overload of the
+itself hashable via `boost::hash<X>` by defining an appropriate overload of the
 function `hash_value`.
 
 See [the documentation of the library](https://www.boost.org/libs/container_hash)

benchmark/Jamfile

@@ -0,0 +1,18 @@
+
+# Copyright Daniel James 2005. Use, modification, and distribution are
+# subject to the Boost Software License, Version 1.0. (See accompanying
+# file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+import testing ;
+
+project :
+  requirements
+    <library>/boost/container_hash//boost_container_hash
+    <library>/boost/core//boost_core
+    <library>/boost/unordered//boost_unordered
+  ;
+
+link char_seq.cpp ;
+link unordered.cpp ;
+link unordered_flat.cpp : <cxxstd>11:<build>no <cxxstd>14:<build>no ;
+link word_count.cpp : <cxxstd>11:<build>no <cxxstd>14:<build>no ;

benchmark/char_seq.cpp

@@ -79,11 +79,11 @@ int main()
 
             if( k & 1 )
             {
-                sprintf( buffer, "prefix_%llu_suffix", k );
+                std::snprintf( buffer, sizeof( buffer ), "prefix_%llu_suffix", k );
             }
             else
             {
-                sprintf( buffer, "{%u}", static_cast<unsigned>( k ) );
+                std::snprintf( buffer, sizeof( buffer ), "{%u}", static_cast<unsigned>( k ) );
             }
 
             v.push_back( buffer );

benchmark/unordered.cpp

@@ -356,11 +356,11 @@ int main()
 
             if( k & 1 )
             {
-                sprintf( buffer, "prefix_%llu_suffix", k );
+                std::snprintf( buffer, sizeof( buffer ), "prefix_%llu_suffix", k );
             }
             else
             {
-                sprintf( buffer, "{%u}", static_cast<unsigned>( k ) );
+                std::snprintf( buffer, sizeof( buffer ), "{%u}", static_cast<unsigned>( k ) );
             }
 
             v.push_back( buffer );

doc/hash/intro.adoc

@@ -44,7 +44,7 @@ Out of the box, `boost::hash` supports
 * `std::variant`, `std::monostate`.
 
 `boost::hash` is extensible; it's possible for a user-defined type `X` to make
-iself hashable via `boost::hash<X>` by defining an appropriate overload of the
+itself hashable via `boost::hash<X>` by defining an appropriate overload of the
 function `hash_value`. Many, if not most, Boost types already contain the
 necessary support.
 

doc/hash/recent.adoc

@@ -8,6 +8,14 @@ https://www.boost.org/LICENSE_1_0.txt
 = Recent Changes
 :idprefix: recent_
 
+== Boost 1.92.0
+
+* Hashing a valueless `std::variant` instance no longer throws.
+
+== Boost 1.89.0
+
+* Added the `hash_is_avalanching` trait class.
+
 == Boost 1.84.0
 
 * {cpp}03 is no longer supported.

doc/hash/reference.adoc

@@ -1,7 +1,7 @@
 ////
 Copyright 2005-2008 Daniel James
 Copyright 2022 Christian Mazakas
-Copyright 2022 Peter Dimov
+Copyright 2022, 2025 Peter Dimov
 Distributed under the Boost Software License, Version 1.0.
 https://www.boost.org/LICENSE_1_0.txt
 ////
@@ -44,6 +44,8 @@ template<class It> std::size_t hash_range( It first, It last );
 template<class It> void hash_unordered_range( std::size_t& seed, It first, It last );
 template<class It> std::size_t hash_unordered_range( It first, It last );
 
+template<class Hash> struct hash_is_avalanching;
+
 } // namespace boost
 ----
 
@@ -572,6 +574,56 @@ where `x` is the currently contained value in `v`.
 Throws: ::
 `std::bad_variant_access` when `v.valueless_by_exception()` is `true`.
 
+== <boost/container_hash/{zwsp}hash_is_avalanching.hpp>
+
+Defines the trait `boost::hash_is_avalanching`.
+
+[source]
+----
+namespace boost
+{
+
+template<class Hash> struct hash_is_avalanching;
+
+} // namespace boost
+----
+
+=== hash_is_avalanching<Hash>
+
+[source]
+----
+template<class Hash> struct hash_is_avalanching
+{
+    static constexpr bool value = /* see below */;
+};
+----
+
+`hash_is_avalanching<Hash>::value` is:
+
+* `false` if `Hash::is_avalanching` is not present,
+* `Hash::is_avalanching::value` if this is present and convertible at compile time to a `bool`,
+* `true` if `Hash::is_avalanching` is `void` (this usage is deprecated),
+* ill-formed otherwise.
+
+A hash function is said to have the _avalanching property_ if small changes
+in the input translate to large changes in the returned hash code
+&#8212;ideally, flipping one bit in the representation of the input value results
+in each bit of the hash code flipping with probability 50%. Libraries
+such as link:../../../unordered/index.html[Boost.Unordered] consult this trait
+to determine if the supplied hash function is of high quality.
+`boost::hash` for `std::basic_string<Ch>` and `std::basic_string_view<Ch>`
+has this trait set to `true` when `Ch` is an integral type (this includes
+`std::string` and `std::string_view`, among others).
+Users can set this trait for a particular `Hash` type by:
+
+* Inserting the nested `is_avalanching` typedef in the class definition
+if they have access to its source code.
+* Writing a specialization of `boost::hash_is_avalanching`
+for `Hash`.
+
+Note that usage of this trait is not restricted to hash functions produced
+with Boost.ContainerHash.
+
 == <boost/container_hash/{zwsp}is_range.hpp>
 
 Defines the trait `boost::container_hash::is_range`.

include/boost/container_hash/hash.hpp

@@ -1,5 +1,5 @@
 // Copyright 2005-2014 Daniel James.
-// Copyright 2021, 2022 Peter Dimov.
+// Copyright 2021, 2022, 2025 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
@@ -11,6 +11,7 @@
 #define BOOST_FUNCTIONAL_HASH_HASH_HPP
 
 #include <boost/container_hash/hash_fwd.hpp>
+#include <boost/container_hash/hash_is_avalanching.hpp>
 #include <boost/container_hash/is_range.hpp>
 #include <boost/container_hash/is_contiguous_range.hpp>
 #include <boost/container_hash/is_unordered_range.hpp>
@@ -454,7 +455,11 @@ namespace boost
         std::size_t seed = 0;
 
         hash_combine( seed, v.index() );
-        std::visit( [&seed](auto&& x) { hash_combine(seed, x); }, v );
+
+        if( !v.valueless_by_exception() )
+        {
+            std::visit( [&seed](auto&& x) { hash_combine(seed, x); }, v );
+        }
 
         return seed;
     }
@@ -557,19 +562,15 @@ namespace boost
 
 #endif
 
-    // boost::unordered::hash_is_avalanching
+    // hash_is_avalanching
 
-    namespace unordered
+    template<class Ch> struct hash_is_avalanching< boost::hash< std::basic_string<Ch> > >: std::is_integral<Ch> {};
-    {
-        template<class T> struct hash_is_avalanching;
-        template<class Ch> struct hash_is_avalanching< boost::hash< std::basic_string<Ch> > >: std::is_integral<Ch> {};
 
 #if !defined(BOOST_NO_CXX17_HDR_STRING_VIEW)
 
-        template<class Ch> struct hash_is_avalanching< boost::hash< std::basic_string_view<Ch> > >: std::is_integral<Ch> {};
+    template<class Ch> struct hash_is_avalanching< boost::hash< std::basic_string_view<Ch> > >: std::is_integral<Ch> {};
 
 #endif
-    } // namespace unordered
 
 } // namespace boost
 

include/boost/container_hash/hash_fwd.hpp

@@ -1,5 +1,5 @@
 // Copyright 2005-2009 Daniel James.
-// Copyright 2021, 2022 Peter Dimov.
+// Copyright 2021, 2022, 2025 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
@@ -32,6 +32,8 @@ template<class It> std::size_t hash_range( It, It );
 template<class It> void hash_unordered_range( std::size_t&, It, It );
 template<class It> std::size_t hash_unordered_range( It, It );
 
+template<class Hash> struct hash_is_avalanching;
+
 } // namespace boost
 
 #endif // #ifndef BOOST_FUNCTIONAL_HASH_FWD_HPP

include/boost/container_hash/hash_is_avalanching.hpp

@@ -0,0 +1,57 @@
+// Copyright 2025 Joaquin M Lopez Munoz.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#ifndef BOOST_HASH_HASH_IS_AVALANCHING_HPP_INCLUDED
+#define BOOST_HASH_HASH_IS_AVALANCHING_HPP_INCLUDED
+
+#include <type_traits>
+
+namespace boost
+{
+namespace hash_detail
+{
+
+template<class... Ts> struct make_void
+{
+    using type = void;
+};
+
+template<class... Ts> using void_t = typename make_void<Ts...>::type;
+
+template<class IsAvalanching> struct avalanching_value
+{
+    static constexpr bool value = IsAvalanching::value;
+};
+
+// may be explicitly marked as BOOST_DEPRECATED in the future
+template<> struct avalanching_value<void>
+{
+    static constexpr bool value = true;
+};
+
+template<class Hash, class = void> struct hash_is_avalanching_impl: std::false_type
+{
+};
+
+template<class Hash> struct hash_is_avalanching_impl<Hash, void_t<typename Hash::is_avalanching> >:
+    std::integral_constant<bool, avalanching_value<typename Hash::is_avalanching>::value>
+{
+};
+
+template<class Hash>
+struct hash_is_avalanching_impl<Hash, typename std::enable_if< ((void)Hash::is_avalanching, true) >::type>
+{
+  // Hash::is_avalanching is not a type: we don't define value to produce
+  // a compile error downstream
+};
+
+} // namespace hash_detail
+
+template<class Hash> struct hash_is_avalanching: hash_detail::hash_is_avalanching_impl<Hash>::type
+{
+};
+
+} // namespace boost
+
+#endif // #ifndef BOOST_HASH_HASH_IS_AVALANCHING_HPP_INCLUDED

test/CMakeLists.txt

@@ -1,4 +1,4 @@
-# Copyright 2018, 2019, 2021, 2022 Peter Dimov
+# Copyright 2018, 2019, 2021, 2022, 2025 Peter Dimov
 # Distributed under the Boost Software License, Version 1.0.
 # See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt
 
@@ -7,6 +7,6 @@ include(BoostTestJamfile OPTIONAL RESULT_VARIABLE HAVE_BOOST_TEST)
 if(HAVE_BOOST_TEST)
 
 boost_test_jamfile(FILE Jamfile.v2
-  LINK_LIBRARIES Boost::container_hash Boost::core Boost::utility Boost::unordered)
+  LINK_LIBRARIES Boost::container_hash Boost::core Boost::utility)
 
 endif()

test/Jamfile.v2

@@ -1,5 +1,5 @@
 # Copyright 2005-2012 Daniel James.
-# Copyright 2022 Peter Dimov
+# Copyright 2022, 2025 Peter Dimov
 # Distributed under the Boost Software License, Version 1.0.
 # https://www.boost.org/LICENSE_1_0.txt
 
@@ -119,10 +119,9 @@ run is_described_class_test3.cpp
 run described_class_test.cpp
   : : : <warnings>extra ;
 
-run hash_is_avalanching_test.cpp
+run hash_is_avalanching_test.cpp ;
-  /boost/unordered//boost_unordered ;
+run hash_is_avalanching_test2.cpp ;
-run hash_is_avalanching_test2.cpp
+run hash_is_avalanching_test3.cpp ;
-  /boost/unordered//boost_unordered ;
 
 run hash_integral_test2.cpp ;
 
@@ -136,3 +135,5 @@ run hash_tuple_like_test2.cpp
 
 run is_range_test4.cpp ;
 run hash_container_test2.cpp ;
+
+run hash_variant_test2.cpp ;

test/hash_float_test.hpp

@@ -33,6 +33,11 @@
 #pragma GCC diagnostic ignored "-Wfloat-equal"
 #endif
 
+#if defined(__clang__)
+// P2614 deprecates has_denorm in C++23
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+#endif
+
 char const* float_type(float*) { return "float"; }
 char const* float_type(double*) { return "double"; }
 char const* float_type(long double*) { return "long double"; }

test/hash_is_avalanching_test.cpp

@@ -1,10 +1,10 @@
-// Copyright 2022 Peter Dimov.
+// Copyright 2022, 2025 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/container_hash/hash.hpp>
+#include <boost/container_hash/hash_is_avalanching.hpp>
 #include <boost/core/lightweight_test_trait.hpp>
-#include <boost/unordered/hash_traits.hpp>
 #include <boost/config.hpp>
 #include <string>
 
@@ -12,7 +12,7 @@ enum my_char { min = 0, max = 255 };
 
 int main()
 {
-    using boost::unordered::hash_is_avalanching;
+    using boost::hash_is_avalanching;
 
     BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::string> > ));
     BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::wstring> > ));

test/hash_is_avalanching_test2.cpp

@@ -1,8 +1,9 @@
-// Copyright 2022 Peter Dimov.
+// Copyright 2022, 2025 Peter Dimov.
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/container_hash/hash.hpp>
+#include <boost/container_hash/hash_is_avalanching.hpp>
 #include <boost/core/lightweight_test_trait.hpp>
 #include <boost/config.hpp>
 #include <boost/config/pragma_message.hpp>
@@ -14,14 +15,13 @@ int main() {}
 
 #else
 
-#include <boost/unordered/hash_traits.hpp>
 #include <string_view>
 
 enum my_char { min = 0, max = 255 };
 
 int main()
 {
-    using boost::unordered::hash_is_avalanching;
+    using boost::hash_is_avalanching;
 
     BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::string_view> > ));
     BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::wstring_view> > ));

test/hash_is_avalanching_test3.cpp

@@ -0,0 +1,38 @@
+// Copyright 2025 Joaquin M Lopez Munoz.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/container_hash/hash_is_avalanching.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+
+struct X
+{
+    using is_avalanching = void;
+};
+
+struct Y
+{
+    using is_avalanching = std::true_type;
+};
+
+struct Z
+{
+    using is_avalanching = std::false_type;
+};
+
+struct W
+{
+};
+
+int main()
+{
+    using boost::hash_is_avalanching;
+
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< X > ));
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< Y > ));
+    BOOST_TEST_TRAIT_FALSE(( hash_is_avalanching< Z > ));
+    BOOST_TEST_TRAIT_FALSE(( hash_is_avalanching< W > ));
+
+    return boost::report_errors();
+}

test/hash_variant_test2.cpp

@@ -0,0 +1,58 @@
+// Copyright 2026 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/container_hash/hash.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX17_HDR_VARIANT)
+
+BOOST_PRAGMA_MESSAGE( "Skipping test because BOOST_NO_CXX17_HDR_VARIANT is defined" )
+int main() {}
+
+#elif defined(BOOST_CLANG_VERSION) && BOOST_CLANG_VERSION < 70100
+
+BOOST_PRAGMA_MESSAGE( "Skipping test because BOOST_CLANG_VERSION < 70100" )
+int main() {}
+
+#else
+
+#include <boost/core/lightweight_test.hpp>
+#include <variant>
+#include <set>
+
+struct X
+{
+    operator std::set<float>() const { throw 5; }
+};
+
+using V = std::variant<std::set<int>, std::set<float>>;
+
+V make_valueless_variant()
+{
+    V v;
+
+    try
+    {
+        v.emplace<1>( X() );
+    }
+    catch( int )
+    {
+    }
+
+    BOOST_TEST( v.valueless_by_exception() );
+
+    return v;
+}
+
+int main()
+{
+    V v1, v2 = make_valueless_variant();
+
+    BOOST_TEST_NE( (boost::hash<V>()( v1 )), (boost::hash<V>()( v2 )) );
+
+    return boost::report_errors();
+}
+
+#endif